Note that new subflows MUST NOT be established (using
the process documented in Section 3.2) until a Digital Signature
Standard (DSS) option has been successfully received across the path
(as documented in Section 3.3).

It should say:

Note that new subflows MUST NOT be established (using
the process documented in Section 3.2) until a Data Sequence
Signal (DSS) option has been successfully received across the path
(as documented in Section 3.3).

Notes:

In this document DSS is indicated as short for both "Digital Signature Standard" and "Data Sequence Signal". I guess the reference to "Digital Signature Standard" was a mistake.

MPTCP.Checksum (flag): This flag is set to true if at least one of
the hosts has set the C bit in the MP_CAPABLE options exchanged
during connection establishment, and is set to false otherwise.
If this flag is set, the checksum must be computed in all DSS
options.

It should say:

MPTCP.Checksum (flag): This flag is set to true if at least one of
the hosts has set the A bit in the MP_CAPABLE options exchanged
during connection establishment, and is set to false otherwise.
If this flag is set, the checksum must be computed in all DSS
options.

Notes:

Checksum is not on bit "C", but instead on bit "A". There are no other instances of referring on bit "C" as the checksum. Bit "C" is unassigned in this version.